customs duty vs duty

customs duty and duty both are nouns.

Word	Noun	Adjective	Verb	Adverb
customs duty	Yes	No	No	No
duty	Yes	No	No	No

As nouns, duty is a hypernym of customs duty; that is, duty is a word with a broader meaning than customs duty:

Other hypernyms of customs duty include tariff.

customs duty (noun)	duty (noun)
money collected under a tariff	the social force that binds you to the courses of action demanded by that force
	work that you are obliged to perform for moral or legal reasons
	a government tax on imports or exports